Caption: Radiolarian. Coloured scanning electron micrograph (SEM) of the test (skeleton) of a radiolarian protozoan. Radiolaria are single-celled marine protozoa which make up part of the plankton. They build up their intricately-patterned tests by exuding siliceous minerals. They feed mainly on other protozoa which become trapped in the spines of their tests. Being part of the plankton, they are themselves eaten by an enormous variety of marine life, from other members of the plankton to whales. Large numbers of radiolarian skeletons litter the sea bed; these become fossilised to form minerals like flint. Magnification unknown.